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The Geology of Hong Kong (Interactive On-line)
Sham Chung Rhyolite - Jks

The Sham Chung Rhyolite (Table 6.3) is a thick flow-banded rhyolite, thought to be a sill, that can be traced from Sham Chung southwards to Yung Shue O on the south side of Tolo Channel. The rhyolite thickens southwards from 10 m to 50 m, and is interpreted as intruding coarse ash crystal tuff of the Shing Mun Formation. The rhyolite contains phenocrysts of bipyramidal quartz and subordinate euhedral alkali feldspar and plagioclase set in a fine-grained, grey to dark grey, aphanitic matrix, containing minor biotite and accessory minerals of zircon, allanite and Fe-oxide. A U–Pb age of 146.6 ± 0.2 Ma has been obtained for the rhyolite (GEO, unpublished data).

A further rhyolite, that appears to be of similar composition, and may be of the same age, occurs at the top of the Lai Chi Chong Formation. However, it is uncertain whether this rhyolite is a sill, in which case it could be part of the Sham Chung Rhyolite, or a lava, in which case it would be part of the Lai Chi Chong Formation.

Based on whole rock geochemistry, up to three rhyolite units within coarse ash tuff of the Shing Mun Formation forming the summits of Ma On Shan and Ngau Ngok Shan are now considered to represent sills belonging to the Sham Chung Rhyolite (Campbell et al., in press).